The development of ever-decreasing size radio transceivers and ever-increasing capacity demands in recent years has favored the emergence of small-size antenna arrays. Compared to a single antenna, an antenna array has enhanced performance features, such as, interference rejection and beam steering without physically moving the aperture. The higher transmission rates, increasing number of users and other new demands placed on the antenna arrays render addressing cross coupling effects among antenna elements even more important.
An antenna array as illustrated in FIG. 1, generally consists of multiple closely spaced antenna elements (or columns) #1, #2, . . . , #n, typically having a distance d of about 0.5 wavelength in-between antenna elements (which distance for radio communication system frequencies of 0.5-5 GHz is in the range of 3-30 cm). The propagation direction of interest is perpendicular (i.e., y-direction) on the plane (i.e., the plane including the x and z axes) of the antenna elements #1, #2, . . . , #n.
Mutual coupling is an electromagnetic phenomenon which occurs between spatially close electromagnetic radiating elements. Due to the antenna elements' closeness, the effects of mutual coupling in an antenna array may be significant. When an antenna element transmits an electromagnetic signal, resonating neighboring elements (or columns) radiate energy according to the transmitted signal. Similarly, when an antenna element (or column) receives an electromagnetic signal, a portion of the energy of the received signal is re-radiated to the neighboring elements (or columns). In many different areas which use antenna arrays, e.g., from the conventional use of antennas to their modern employment in such exotic areas as multiple-input multiple-output (MIMO) systems, diversity systems, medical imaging, and radar systems, the manner of taking into consideration these mutual coupling effects is important.
Classical theoretical calculations can be used to determine an expected beam pattern in a plane perpendicular to the antenna array plane in the direction of interest. Such calculations are used in designing antenna arrays, and typically assume that the effects of mutual coupling are either non-existent or are so small that they can be neglected. Unfortunately, this assumption becomes increasingly inaccurate as the array elements are spaced closer together and operate in a live air environment. Recently, many attempts have been made to reduce or to compensate for mutual coupling effects.
Some methods which have been proposed to account for these mutual coupling generally result in a compromise design. The compromise design is achieved by repeated iterations and testing. Tradeoffs that impact critical antenna specifications are unavoidable due to design changes implemented to avoid mutual coupling. Typically, the design variables employed to account for the mutual coupling include the radiating element design, the column spacing, the inter-column offsets and the beam formers. This conventional approach to accounting for the mutual coupling of individual elements of an antenna array has the disadvantage that these methods are approximations, and in the end, in spite of the longer antenna design time, the antenna arrays remain plagued by residual mutual coupling impairments.
An accurate determination of mutual coupling coefficients is not straightforward. Although receiving mutual coupling coefficients and transmitting mutual coupling coefficients are expected to be similar, they may differ significantly due to different current distributions that occur on the antenna elements (or columns). Direct measurement of mutual coupling is impractical for a typical antenna array design.
Additionally, although the mutual coupling effects are the most frequently considered cross-coupling effects, these effects may not be the only effects which impair performance. Thus, taking into account mutual coupling effects (which may be measured or estimated) may still leave other quality degrading effects unaccounted for.
Accordingly, it would be desirable to provide devices, systems and methods that avoid the afore-described problems and drawbacks.